<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    function observer(target) {
      if (typeof target !== 'object' || target == null) {
        return target
      }

      for (let key in target) {
        defineReactive(target, key, target[key])
      }
    }

    function defineReactive(target, key, value) {
      observer(value)
      Object.defineProperty(target, key, {
        get() {
          return value
        },
        set(newValue) {
          if (newValue !== value) {
            observer(newValue)
            value = newValue
          }
        }
      })
    }
  </script>
</body>

</html>